(a) Field of the Invention
The present invention relates to a hydraulic cylinder, and more specifically to the oil return loop of a hydraulic cylinder which facilitates the movement of the oil seal ring with the piston, so that the service life of the oil seal ring is greatly prolonged.
(b) Description of the Prior Art
A regular hydraulic cylinder has a safety oil release hole between the inner cylinder and the outer cylinder. Through the safety oil release hole, saturated hydraulic oil flows from the inner cylinder to the outer cylinder when the piston is extended out of the inner cylinder, so as to prevent an explosion of the hydraulic cylinder due to an overload. U.S. Pat. No. 4,077,608 discloses a design of safety oil return loop, in which an oil return groove is provided at the top of the piston. When the piston is moved with the piston rod to the top end of the inner cylinder, the oil return groove imparts a passage to guide saturated oil from the inner cylinder to the outer cylinder. However, because the oil seal ring which is mounted around the piston tends to be deformed to block the oil return groove when the piston is moved to the top end of the inner cylinder, saturated hydraulic oil cannot be smoothly guided out of the inner cylinder to the outer cylinder.